This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Implantable Cardioverter Defibrillators (ICDs) are implantable medical devices (IMDs) proven to prevent sudden cardiac death (SCD) due to ventricular arrhythmia and thus prolong life. ICDs perform therapy by sensing arrhythmia, and delivering timed anti-tachycardial pacing (ATP) or in cases where this does not work, they defibrillate the heart by delivering a shock to convert the heart to normal sinus rhythm.
ICDs have at least one lead implanted in a chamber of the heart, and up to 3 leads for CRT+ICD devices. The majority of these devices rely on accurate classification of the electrical signals (intracardiac ECGs) that are sensed on one or more electrodes implanted in various chambers of the heart. If an electrical arrhythmia is detected, the ICD determines whether to shock or to pace the heart to convert the rhythm back to normal sinus rhythm before hemodynamic instability occurs. Alternatively, sinus tachycardia can occur which would be neither paced nor shocked, but observed. These determinations are currently made without knowledge of the hemodynamic status of the heart, which can cause the device to misclassify some arrhythmias that do not need treatment causing an “inappropriate shock”. In contrast, when physicians treat patients having ongoing arrhythmias with congestive heart failure from dilated and failing left ventricles, they are trained to determine hemodynamic parameters such as blood pressure or cardiac output (stroke volume). If the blood pressure and cardiac output (stroke volume) are adequate, they are trained to attempt conservative measures to terminate the arrthymia such as pacing or iv administration of medications. However, if the blood pressure or cardiac output (stroke volume) are unstable and too low, then physicians are trained to administer cardioversion (electrical shocks).
It has been reported that anywhere from 20-35% of patients who receive an ICD will experience an “appropriate” shock within the first 1-3 years of receiving the implant. Around a third of patients who receive an ICD experience an “inappropriate” shock, indicating that therapy was delivered unnecessarily, and the remaining third do not receive a shock (2). Receiving shock therapy is associated with a two- to five-fold increase in mortality in these patients, so the elimination of unnecessary shocks has become an important focus in ICD research within the last decade. The main reasons for a device delivering an inappropriate shock are:
1. An atrial fibrillation (AF) or other supra-ventricular tachycardia (SVT) event including sinus tachycardia which is misclassified as a more serious rhythm such as ventricular tachycardia or fibrillation.
2. The ICD misclassifies the rate of the heart because it counts the heart rate twice for every beat (called oversensing) by misreading the QRS complex and T wave.
3. Mechanical problems such as lead fracture, or dislodgement of the lead.
The first two of these problems occur because the electrical system of the heart (ECG) is being used to determine cardiac hemodynamics. Clearly, a more direct measurement of hemodynamics would be more useful.